Strains of the genus Saccharomyces are used widely in the industry for brewing, distilling, baking, bioethanol production and various other applications. Saccharomyces cerevisiae is one of the most widely used microorganisms in industrial applications in view of it's ability to convert sugars such as glucose, fructose, mannose, maltose and sucrose to cell mass, and fermenting these sugars to ethanol. Strains of Saccharomyces cerevisiae are used in the fuel industry in view of their ability to rapidly convert sugars into ethanol. Saccharomyces cerevisiae has a better tolerance towards fermentation inhibitors and ethanol compared to bacteria and other yeast species.
Unlike bacteria and several yeast species, wild-type Saccharomyces cerevisiae is not able to use pentoses such as xylose and arabinose as carbon source. The ability of Saccharomyces cerevisiae to grow on abundant carbon sources such as side streams and residual material from other processes, such as agricultural residual material from e.g. maize and bagasse, and residual material from e.g. paper manufacture, is of great environmental, but also economical, value. Agricultural residual material and hardwood derived streams comprise a rather large fraction of hemicellulose, which is built from many different sugar monomers. For instance, besides glucose, these sugar monomers can include xylose, mannose, galactose, rhamnose and arabinose. Glucose and xyloseare the sugar monomers that are present in the largest amount and thus represents an important carbon source for the manufacturing of ethanol using yeasts, providing a huge economic and environmental advantage. The abundance of xylose in mentioned materials and the possibility to use yeasts, such as Saccharomyces cerevisiae, to produce ethanol using xylose as carbon source has led to intense research within this field of technology. The conversion of xylose has however sometimes been poor resulting in a poor ethanol production. Further the production of the byproduct xylitol has been rather large.
Genes encoding enzymes giving the ability to use xylose as carbon source have previously been introduced in Saccharomyces cerevisiae. 
A comparison using a blast search of the amino acid sequences Xyl1 (xylose reductase) and Xyl2 (xylitol dehydrogenase) from Scheffersomyces stipitis with all proteins in S. cerevisiae show that the closest homologues are Gre3p (Sc) E-value: 3E-100 with Xyl1, and Xyl2 is closest to Sor1p, Sor2 E-value 1.4E-86 to 1 E-77. The smaller the E value the larger homology of the genes. Thus, a large homology between sequences is observed. EP 1 282 686 discloses recombinant Saccharomyces cerevisiae strains having incorporated genes for the enzymes xylose reductase, xylitol dehydrogenase and xylulokinase as well as having been subjected to a specific mutation. Said strains have the ability to ferment lignocellulose raw materials to ethanol. The strain deposited in Ep 1 282 686 is CBS 102679 (TMB3400, Taurus01) is generally recognised to be efficient in the prior art. The ethanol produced by the strain CBS 102679 has been considered very good compared to other prior art recombinant yeasts, but there is also a production of the undesirable byproduct xylitol. Therefore, there is still a need within the art to provide new strains of Saccharomyces cerevisiae having an even better ethanol production, better xylose conversion as well as lower xylitol production.
WO2012/067572 discloses Saccharomyces cerevisiae strains Taurus03 with deposit number CBS128138, Taurus04 with deposit number CBS 128139, Taurus07 with deposit number CBS128140, Taurus10 with deposit number CBS128141, which all are xylose fermenting yeast strains producing beneficial ethanol yields.
The β-lactamase gene is included in at least the strains Taurus01, Taurus04 and Taurus07 as mentioned above. The authorities in the US do not allow the use of Saccharomyces cerevisiae strains, which contain the β-lactamase gene, in larger production facilities in view of risk for genetic transfer of the gene to another organism which then potentially can obtain antibiotic resistance.
There is still a need within the technical field to provide robust Saccharomyces cerevisiae strains providing high ethanol yields from both 5- and 6-carbon sugars and in addition exhibiting low by-product yields of eg xylitol. Saccharomyces cerevisiae strains not having the above mentioned β-lactamase gene in the genome are especially needed.